Considerable evidence indicates that differences in metabolic processing of chemical toxins are critical in determining both species and organ sensitivity to individual compounds. The microsomal cytochrome P-450 system catalyzes the oxidation of a variety of substrates and consists of a family of cytochrome P-450 isoenzymes supported by other reductase proteins. The objectives of the present study are to characterize the cytochrome P-450 system using the carcinogen acetylaminofluorene which is oxidized in the 1, 3, 5, 7, 9 position on the fluorene ring and on the nitrogen. Further, once the pattern of acetylaminofluorene metabolism has been characterized the effects of various inducers and inhibitors of the microsomal monooxygenase system will be investigated. Studies to date have characterized acetylaminofluorene metabolism in control rat liver microsomes and in rat microsomes following phenobarbital and TCDD induction. These studies indicated that at least 2 cytochrome P-450 isoenzymes were responsible for the formation of the 1, 3, 5, 7 and 9-OH acetylaminofluorene while N-hydroxylation appeared to only involve one distinguishable isoenzyme. The activation of acetylaminofluorene (N-hydroxylation) was induced by TCDD whereas only 7-hydroxylation was induced by phenobarbital. Further studies have investigated acetylaminofluorene metabolism in human liver microsomes and rabbit liver microsomes. Studies presently underway involve correlating the kinetics of acetylaminofluorene metabolism in rabbit liver microsomes to that of purified forms of cytochrome P-450. The characterization of lung monooxygenase is also being pursued using this substrate.